Almost since the invention of the transistor workers in the field have pursued the goal of greater and greater operating speed of such devices. The success of this endeavor has been spectacular, resulting in commercial devices capable of operation in the GHz regime. However, the quest continues unabated, and structures have been proposed that seem capable of substantially greater speed yet. Among such devices are transistors comprising a two-dimensional electron gas (2DEG) in a quantum well (QW) or an inversion layer. By a 2DEG we mean an assembly of electrons that are frozen in a quantized energy level for one spatial dimension but are free to move in the other two spatial dimensions. Exemplary of devices that comprise a 2DEG is the hot electron unipolar transistor disclosed in U.S. Pat. No. 4,691,215, incorporated herein by reference. For a review of the properties of a 2DEG, see, for instance, T. Ando et al., Reviews of Modern Physics, Vol. 54(2), page 437 (1982), (especially pp. 437-458), also incorporated herein by reference.
Even though a variety of potentially very high speed transistor designs are already known to the art, there exists still considerable interest in the discovery of novel approaches, especially approaches based on previously unrecognized phenomena or characteristics, since such novel devices may have advantages over prior art devices. In any case, the availability of a multiplicity of potential designs makes it easier for the device designer to pick a device most suitable for any given task.
Most transistors known to the prior art comprise a control region (base or gate) between a charge carrier emission region (emitter or source) and a charge carrier collection region (collector or drain). However, recently a transistor structure has been proposed that has a different arrangement of the three regions. In particular, the proposed transistor comprises an emitter that is separated from a collector only by a very thin potential barrier, with the collector being intermediate the emitter and the base. See, A. R. Bonnefoi et al, Applied Physics Letters, Vol. 47(8), pp. 888-890, (1985).
According to the published description of the above device, the collector region is a lightly doped n-type QW, separated from the base by a relatively thick lightly doped barrier layer. As can be seen from FIG. 1b of the reference, at equilibrium the Fermi level in the collector QW is substantially below the lowest energy state of the 2DEG. Application of an electric field between base and emitter is said to modify the positions of the sub-bands in the well with respect to the emitter Fermi level and thus modulates the tunneling current from the emitter into the collector QW. Because the operation of the prior art device depends on the modulation of the quasi-stationary states in the QW by the base electric field to produce transistor I-V characteristics, the prior art device is referred to as the Stark effect transistor.